export function debounce (func, wait, immediate) {
  let timeout;
  let args;
  let context;
  let timestamp;
  let result;

  const later = () => {
    // 据上一次触发时间间隔
    const last = +new Date() - timestamp

    // 上次被包装函数被调用时间间隔last小于设定时间间隔wait
    if (last < wait && last > 0) {
      timeout = setTimeout(later, wait - last)
    } else {
      timeout = null
      // 如果设定为immediate===true，因为开始边界已经调用过了此处无需调用
      if (!immediate) {
        result = func.apply(context, args)
        if (!timeout) {
          context = null
          args = []
        }
      }
    }
  }

  return ( ...params ) => {
    context = this
    timestamp = +new Date()
    const callNow = immediate && !timeout
    // 如果延时不存在，重新设定延时
    if (!timeout) {
      timeout = setTimeout(later, wait)
    }
    if (callNow) {
      result = func.apply(context, params)
      context = null
      params = null
    }

    return result
  }
}
